GB1569514A

GB1569514A - Pesticidal composition and process for its preparation

Info

Publication number: GB1569514A
Application number: GB4291/78A
Authority: GB
Original assignee: Freyberg Dr Werner Chemische Fabrik
Current assignee: Freyberg Dr Werner Chemische Fabrik
Priority date: 1977-02-08
Filing date: 1978-02-02
Publication date: 1980-06-18
Also published as: ZA777233B; JPS53124616A; RO76148A; CA1103151A; ES466775A1; CH645245A5; IL53986A0; IL53986A; JPS6233203B2; DE2705228C2; DD136569A5; MX5333E; DE2705228A1; AR219101A1

Abstract

A metal phosphide which is readily hydrolysable and suitable for controlling pests can be made hydrophobic without preventing contact with water vapour. The agent used for rendering the metal phosphide hydrophobic is an organosilicon compound which has at least one hydrogen atom, halogen atom or a hydroxyl or alkoxy group which is bonded directly to the silicon atom.

Description

(54) PESTICIDAL COMPOSITION AND PROCESS FOR ITS PREPARATION (71) We, DR. WERNER FREYBERG CHEMISCHE FABRIK, Delicia Nachf, 6941 Laudenbach, Germany, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a pesticidal composition adapted to split off hydrogen phosphide by the reaction of easily hydrolysable phosphides with moisture. The metal phosphides available for pesticidal purposes are mainly calcium phosphide, aluminium phosphide and magnesium phosphide. These phosphides suffer from the disadvantage that they react more or less violently on contact with water. A simultaneously occurring rise in temperature may therefore result in the ignition of the hydrogen phosphide generated.

Attempts have been made for some time to overcome this disadvantage by the addition of hydrophobic substances.

Apart from the known ways of rendering the phosphides hydrophobic such as by addition of metal soaps. paraffin, resins and waxes, the use of silicones for such purposes has also been described. Thus, in accordance with German Auslegeschrift No. 1 023 265, an addition of silicone is said to delay considerably the gas release of phosphide on admission of moisture. However, this is not desirable in practice. On the contrary, it is preferred to suppress the reaction of the phosphide with liquid water by rendering the phosphide hydrophobic, while not preventing generation of hydrogen phosphide gas resulting from the action of atmospheric moisture.

In addition German Offenlegungsschrift No. 1 542 877 describes the provision of an adequate protection against moisture by the use of liquid hydrophobic agents while not interfering with the useful properties of the phosphides. More particularly this is said to be attainable by the application of methyl or phenyl silicone oils onto the surface of moulded bodies, e.g. tablets. of the phosphides. Apart from the fact that the process is limited to moulded bodies, it has been found that the employment of those silicone oils does not impact a satisfactory hydrophobic coating to the tablets.

The addition of about 3% methyl or phenyl silicone oil does not result in imparting adequate hydrophobic properties to the phosphides. On the other hand, larger additions interfere considerably with the flowing and pressing properties of the mixture, preventing the normal further processing or use.

Disadvantages of the use of other known hydrophobic agents are, for example, the fact that when using metal soaps, comparatively large amounts of such additives are necessary in order to attain adequate hydrophobic properties. This imposes limitations on the addition of other important auxiliaries which are necessary for suppressing self-ignition properties.

The flowing properties and pressing properties of the mixture are adversely affected by large amounts of additives. Paraffins, synthetic resins and waxes frequently cannot be applied easily; they must be pretreated (by grinding, melting or dissolving). Here as well, comparatively large amounts of additive are necessary to obtain adequate hydrophobic properties.

There still exists a need to render hydrophobic in a simple manner and to an adequate extent readily hydrolisable metal phosphides intended for pesticidal purposes without impeding the desired access of water vapour, while keeping the necessary amount of hydrophobic agent as low as possible.

In accordance with one aspect of the invention there is provided a metal phosphide composition including a particulate hydrolisable metal phosphide of a type suitable for pest control purposes and which on contact with moisture will release hydrogen phosphide, and a silicon-organic compound having directly bonded to silicon one or more hydrogen atoms, halogen atoms, or groups of formula -OR1, wherein R1 represents a hydrogen atom or a hydrocarbon moiety, the silicon-organic compound acting as a hydrophobic agent which is capable of rendering the metal phosphide hydrophobic.

When R1 represents an alkyl moiety it preferably is a lower alkyl moiety, i.e. an alkyl group having a molecular weight not exceeding 150. A preferred meaning of R, is methyl, ethyl or phenyl.

In accordance with a preferred embodiment of the invention, the silicon-organic compound has the general formula:

wherein each of the moieties R. represents an alkyl group, preferably a low molecular alkyl group, in particular the methyl group or an aryl group, in particular a phenyl group, the groups x, y and z have the same meaning as R2 or represent hydrogen, with the proviso that not all of the said groups x, y and z have the former meaning, whilst n, m and o each represent a whole number, the sum total of which is between 10 and 1000, preferably between about 20 and 100.

Preferably, the individual substituents R. are all the same.

It is particularly advantageous to use methyl hydrogenpolysiloxanes in the form of oils of low viscosity.

Preferably the silicon-organic compounds have a viscosity of about 10 to 1000 cP at room temperature (i.e. 20"C).

These silicon oils comprise reactive bonds which are capable of entering into substitution reactions. Thus the Si-H-bond for example reacts with compounds containing hydroxyl groups in accordance with the following equation: Si-H + HOR < Si-O-R + H2 wherein basic substances such as ammonia and amines accelerate the reaction.

Preferably the hydrophobic agent is uniformly distributed throughout in the composition or in respect of the phosphide particles.

In the preferred compositions the phosphide is calcium phosphide, magnesium phosphide or aluminium phosphide, most preferably aluminium phosphide.

Preferably the or a substantial part of the hydrophobic agent is chemically bonded to surfaces of the particulate phosphide. Preferably the particulate phosphide is technical grade aluminium phosphide or the type conventionally used in pesticidal aluminium phosphide preparations.

In certain preferred embodiments the presence of hydrophobic agent is substantially restricted to the surfaces of the particulate phosphide. In this manner the hydrophobic effect is utilised particularly economically because the hydrophobic agent becomes effective where it is actually needed in the composition and is not wasted on parts of the composition requiring no protection against liquid water.

Preferred compositions are rendered hydrophobic with from 0.1 to 3% by weight of the hydrophobic agent, more preferably with from 0.2 to 0.5% by weight of the hydrophobic agent.

According to a further aspect of the invention there is provided a pest control composition in the form of a mixture comprising particles of a metal phosphide of a type selected from the phosphides of calcium, magnesium and aluminium, a chemical agent for suppressing the self-ignition of hydrogen phosphide selected from ammonium and amine compounds, said chemical agent being in particulate form, and a hydrophobising content of a hydrophobic silicon-organic compound chemically bonded to surfaces of particles of the mixture.

Preferred compositions are in the form of bodies moulded from the mixture of particles by compression. However, the invention is also applicable to the provision of the composition in powder form, e.g. contained in small bags or sachets or other containers of paper or other suitable material pervious to atmospheric moisture, but preferably repellent to liquid water. Such containers are known per se in this art.

Preferably the said chemical agent for suppressing self-ignition of hydrogen phosphide is a mixture of particles of urea and particles of ammonium carbamate.

Also in accordance with the invention there is provided a process for the manufacture of a pesticidal composition as herein described, which comprises mixing the fine particulate phosphide or the mixture of the phosphide and any further required additives with the silicon-organic compound and subjecting the said compound to conditions in which it reacts to hydrophobise components of the mixture.

The invention is based on the surprising observation that excellent results are attained by using silicon-organic compounds of the type defined above to impact hydrophobic properties to metal phosphides. This behaviour can be explained, at least in part, by attributing the hydrophobic effect to a shell of organo-silicon compound formed around the phosphide particles as a result of surface reactions by which the reactive groups of the silicon-organic compounds enter into a strong bond with metal groups which are inevitably present in technical grade phosphides. These hydroxyl moieties are formed in the manufacture of phosphides because of the unavoidable reaction of the individual phosphide crystals with moisture. It is believed that the organopolysiloxane chains become so orientated that the surface of the phosphide particles is screened off by the outwardly directed methyl groups resulting in an excellent hydrophobic effect. The chemical reaction which may take place when the phosphide is being rendered hydrophobic may be accelerated by raising the temperature e.g. to 80 to 200"C, preferably to from 110 to 1500C, and/or by the addition of suitable catalysts. Such catalysts include ammonia, ammonium compounds or amine compounds, e.g. ammonium carbamate or ammonium carbonate, which may be employed in amounts of 0.5 % to 3% by weight based on the metal phosphide. The catalysts can also be organo-metallic compounds; suitable such catalysts are for example compounds of lead. tin, zirconium, cobalt or titanium, examples of particularly suitable such catalysts being zirconiumbutoxide, dibutyletindiluarate and di-noctyltinmaleinate. These catalysts may be employed in amounts of preferably about 1 to 10% by weight based on the silicon-organic compounds.

A special embodiment of the invention is a process in which an excellent and effective hydrophobing of the phosphide is attained by the use of the silicon-organic compound without employing special measures or additives.

Preferably, the metal phosphide-containing pesticidal compositions of the invention contain NH3-generating additives, such as for example urea, ammonium carbonate, ammonium carbamate or amines. These additives are distributed in particulate form uniformly through the composition.

These substances act to accelerate the reaction of the silicon-organic compound so that simple addition of these during the manufacture of the mixture is adequate for the phosphide to attain the desired hydrophobic properties. Immediately after the manufacture the product has hydrophobic properties which are further increased during the course of further processing and storage.

The composition may be pressed into moulded bodies, e.g. tablets or pellets. These have a degree of porosity permitting access of atmospheric moisture to the hydrophobic metal phosphide. No reliance is placed on any "bursting effect" for access of moisture through any water repellent coatings as has been proposed elsewhere. The hydrophobic treatment in accordance with the present invention provides the desired protection against liquid water but has not been observed to impede in any way the desired rate of generation of phosphine due to the hydrolising action of atmospheric moisture.

In accordance with other embodiments of the process, the phosphide is mixed only with the silocon-organic compound, where appropriate with a subordinate catalysing amount of catalyst, until the silicon-organic compound has become wholly or substantially wholly bonded to the phosphide particles which are thereby rendered hydrophobic. Again the further additive(s) may include urea and/or ammonium carbamate in amounts to suppress auto-ignition.

Initially only a catalysing amount of the catalyst substance such as ammonium carbamate is added which may become largely or wholly volatilised during the hydrophobing reaction (the resulting ammonia vapours acting as catalyst as well). For example, the catalysing amount of ammonium carbamate at this stage may be between 0.5 and 3% by weight based on the metal phosphide, whereas the final concentration of additives (e.g. ammoniumcarbamate and urea) is much larger and may be between 10 and 50%, more particularly between 20 and 35%, say 30% of the composition, the final composition then preferably being compressed into moulded bodies, e.g. tablets or pellets.

Accordingly the hydrophobic effect is confined wholly or substantially to the phosphide particles, for effective protection against liquid water without impeding the desired access of water vapour.

It is surprising that very small amounts of silicon-organic compounds suffice in order to render strongly hydrophobic the metal phosphide-containing preparations. In order to attain similar hydrophobic effects, amounts of silicones which are 10 - 30 times greater have to be used, while other solid hydrophobic agents have to be used in quantities which may even be up to 100 times greater.

We have found in order to render hydrophobic conventional phosphide-containing preparations suitable for pesticidal purposes, it is sufficient to use 0.2 - 0.5% of silicon-organic compound defined above, based on the weight of mixture of the phosphide and other additives which may be present. Even smaller additions of only 0.1% result in hydrophobic properties which for many purposes are adequate. In exceptional cases only might it be desirable to employ higher proportions up to about 3% of siliconorganic compound based on the weight of the final product.

Thus according to a further aspect of the present invention there is provided a particulate technical metal phosphide of a type suitable for use in pest control and capable of generating phosphide by reaction with moisture under pest control conditions composed of particles, the surfaces of which have been rendered hydrophobic by the chemical bonding to said surfaces of hydrophobic reaction products on said surfaces of hydrophobic reaction products of a silicon-organic compound comprising directly linked to silicon one or more hydrogen atoms, halogen atoms or groups -OR1, wherein R1 represents hydrogen or a hydrocarbon moiety. The same preferred features apply to the silicon-organic compound as are set out above. Again, the preferred phosphide component is technical phosphide.

The invention also provides a technical metal phosphide, reactive with water to generate phosphide and comprising surface bonded compounds of the general formula MeO- Si wherein Me represents a metal atom of the metal phosphide and the group - Si represents a hydrophobic silicon-organic group. Again the preferred features pertaining to the hydrophobic agent also pertain to the group - Si a in an analogous manner. Thus, in a preferred phosphide the group -SI is represented by a group of the general formula

wherein each of the groups R2 represents an alkyl or aryl moiety. the group x, y and z having the same meaning as R2 or that of hydrogen subject to the condition that not all of group x, y and z have the first mentioned meaning, and wherein n, m and o each represent whole numbers, the sum total of which amounts to between 10 and 1000 and wherein at least one of the group x, y and z represents the group MeO The scope of the invention further includes a method of pest control which comprises exposing material or an environment in which pests are to be controlled to a pesticidal concentration of hydrogen phosphide generated by the action of moisture onto a metal phosphide or preparation or composition as set out above, more particularly wherein the mosture is atmospheric moisture.

In particular embodiments of the method a preparation or composition as set out is added to agricultural produce to be disinfested.

The following Examples and Experiments further illustrate the invention.

Example 1 70 parts of technical grade A1P are finely ground and mixed with 10 parts of finely powdered ammonium carbamate and 19.5 parts grounds urea. During mixing 0.5 parts methyl hydrogen polysiloxane having a viscosity of about 15 cP are added. The total mixing period amounts to 30 minutes. A free-flowing powder is attained which can be pressed into moulded bodies without further addition of further additives.

The properties of such pressed bodies are shown in Tables I and II.

Example 2 99 parts technical grade A1P are mixed intimately in a mixer with 0.9 parts methy-hydrogen polysiloxane for 15 minutes. 0.1 parts zirconium butoxide are then added and mixing is continued for a further 15 minutes.

A dust-free, free-flowing, hydrophobic phosphide is attained.

Example 3 A1P is manufactured by thermal reaction between Al and P. As soon as the aluminium phosphide has cooled down to about 1600C 2 parts methylhydrogenpolysiloxane are added to 98 parts of A1P. The resultant mixture is crudely mixed and then ground.

After the phosphide has cooled down, it has pronounced hydrophobic properties.

Experiment 1 70 parts of the hydrophobic phosphide of Example 2 or 3 respectively are mixed in a known manner with 20 parts urea and 10 parts ammonium carbamate and subsequently pressed into tablets having a diameter of 18 mm.

A tablet having the same composition but produced with an untreated phosphide is used for comparison. In order to demonstrate the water-repellent properties, a drop of water is placed onto the tablet surfaces.

Whilst the tablet according to the prior art reacts immediately with the water, the drop of water remains on the surface of the tablet in accordance with the invention for a prolonged period. A slow reaction commences only after about 10 minutes, which even after 30 minutes has not finished.

Experiment 2 Table 1 illustrates the effect of the use of methyl hydrogen polysiloxane in accordance with the invention. here the average PH3-generation of different metal phosphidecontaining moulded bodies each having a mass of 3g is demonstrated on contact with liquid water at 200C.

For this purpose according to the principle of the pneumatic trough, a measuring cylinder having a volume of 250 ml is filled with water and is placed upside down into a trough filled with water and with the exclusion of air. Once tablets containing hydrolysable phosphides are placed under the measuring cylinder, the amount of gas split off by the water can be observed in terms of ml as a function of time. The smaller the amount of gas split off per unit of time, the stronger has been the hydrophobic effect.

TABLE I Hydrogen phosphide gas generated in ml after 5 min. 10 min. 30 min. 60 min.

70 % technical grade A1P 10 15 42 95 10 % ammonium carb.

19.5 % urea 0.5 % methylhydrogen polysiloxane 70 % A1P techn. grade 40 90 200 250 10 % ammonium carb.

19.5 % urea 0.5 % methylsilicon oil 70 % A1P techn. grade 12 20 150 250 10 % ammoniumcarb.

10 % urea 10 % aluminium stearate The hydrophobic effect produced in accordance with the invention has the additional important advantage of a very good permeability for water vapour. This, in the context of pest control by means of the preparations in accordance with the invention is particularly advantageous in that the gas generation of the phosphide is not impeded, which meets the requirements for preparations having a short effective period of activity.

Experiment 3 Moulded bodies produced in accordance with the invention, as is apparent from Table II, are not impeded in their gas generating properties in contrast to preparations hydrophobised by means of paraffin wax.

For this purpose various tablets were exposed to air having a relative moisture of 40 to 60% at 20"C and were analysed in respect of their residual contents of available hydrogen phosphide based on the initially present amount after the stated intervals.

TABLE II Relative con tent of PH3 (Initial content = 100%) Tablet in accordance with the inven- Tablet hydrophobised tion produced according to example 1 with 8% paraffin wax after 0 hours 100 % 100 % 4 hours 92.0 % 94.1 % 6 hours 73 % 81.1 % 16 hours 20.2 % 45.3 % 24 hours 8.9 % 40.8 % 48 hours 3.0 % 9.5 % A further advantage when employing methylhydrogenpolysiloxanes resides in the very good free-flowing properties of the powder thus rendered hydrophobic and in the mould release properties of the silicone oil which greatlv facilitates the manufacture of pressed bodies. In particular, the otherwise frequently interfering effect of adhesion to pressing tools is avoided.

An additional practical side effect resides in the fact that powdered renders hydrophobic in accordance with the present invention hardly generate dust during processing.

Example 4 An intimate, finely particulate mixture is prepared of 70 parts technical grade aluminium phosphide and 2 parts ammonium carbamate. To this mixture 0.5 parts methyl hydrogen polysiloxane having a viscosity of about 15 cP are added with thorough agitation maintained for 30 minutes at a temperature of about 100"C. To the resulting cooled down mixture, 19.5 parts bv weight finely ground urea are added and further ammonium carbamate so that the final composition contains 10 parts by weight ammonium carbamate. The resulting powder is free-flowing and the aluminium phosphide content thereof is strongly hydrophobic. The powder is suitable for pressing into tablets without further additives. In the pressed tablets the hydrophobic effect is also substantially confined to the surfaces of the aluminium phosphide particles.

Although in the aforegoing Examples the invention has been exemplified with particular reference to aluminium phosphide analogous results are obtained if the known grades of magnesium phosphide or calcium phosphide known for use in pesticidal compositions are employed substantially in analogy to conventional calcium phosphide and magnesium phosphide pesticidal compositions.

WHAT WE CLAIM IS: 1. A metal phosphide composition including a particulate hydrolisable metal phosphide of a type suitable for pest control purposes and which on contact with moisture will release hydrogen phosphide, and a silicon-organic compound having directly bonded to silicon one or more hydrogen atoms, halogen atoms, or groups of formula -OR1, wherein Rl represents a hydrogen atom or a hydrocarbon moiety. the silicon-organic compound acting as a hydrophobic agent which is capable of rendering the metal phosphide hydrophobic.

2. A composition according to Claim 1. the composition having been subjected to a treatment such that some or all of the organo-silicon compound has become chemically bonded to the surface of the particles of metal phosphide thereby rendering the surface of the metal phosphide particles hydrophobic.

3. A composition according to Claim 2, wherein the said treatment includes heating the composition to a temperature of from 80" to 200"C.

4. A composition according to Claim 2 or 3. wherein the said treatment includes the addition of a catalyst for promoting the reaction of the silicon-organic compound with the particles of metal phosphide.

5. A composition according to Claim 4, wherein the catalyst is ammonia, an ammonium compound or an amine compound. or an organo-metallic compound.

6. A composition according to any one of the preceding claims, wherein the hydrocarbon moiety is an alkyl moiety having a molecular weight not exceeding 150.

7. A composition according to any one of the preceding claims, wherein the hydrocarbon moiety is methyl or ethyl or phenyl.

8. A composition according to any one of the preceding claims, wherein the silicon organic compound has the general formula

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. TABLE II Relative con tent of PH3 (Initial content = 100%) Tablet in accordance with the inven- Tablet hydrophobised tion produced according to example 1 with 8% paraffin wax after 0 hours 100 % 100 % 4 hours 92.0 % 94.1 % 6 hours 73 % 81.1 % 16 hours 20.2 % 45.3 % 24 hours 8.9 % 40.8 % 48 hours 3.0 % 9.5 % A further advantage when employing methylhydrogenpolysiloxanes resides in the very good free-flowing properties of the powder thus rendered hydrophobic and in the mould release properties of the silicone oil which greatlv facilitates the manufacture of pressed bodies. In particular, the otherwise frequently interfering effect of adhesion to pressing tools is avoided. An additional practical side effect resides in the fact that powdered renders hydrophobic in accordance with the present invention hardly generate dust during processing. Example 4 An intimate, finely particulate mixture is prepared of 70 parts technical grade aluminium phosphide and 2 parts ammonium carbamate. To this mixture 0.5 parts methyl hydrogen polysiloxane having a viscosity of about 15 cP are added with thorough agitation maintained for 30 minutes at a temperature of about 100"C. To the resulting cooled down mixture, 19.5 parts bv weight finely ground urea are added and further ammonium carbamate so that the final composition contains 10 parts by weight ammonium carbamate. The resulting powder is free-flowing and the aluminium phosphide content thereof is strongly hydrophobic. The powder is suitable for pressing into tablets without further additives. In the pressed tablets the hydrophobic effect is also substantially confined to the surfaces of the aluminium phosphide particles. Although in the aforegoing Examples the invention has been exemplified with particular reference to aluminium phosphide analogous results are obtained if the known grades of magnesium phosphide or calcium phosphide known for use in pesticidal compositions are employed substantially in analogy to conventional calcium phosphide and magnesium phosphide pesticidal compositions. WHAT WE CLAIM IS:

1. A metal phosphide composition including a particulate hydrolisable metal phosphide of a type suitable for pest control purposes and which on contact with moisture will release hydrogen phosphide, and a silicon-organic compound having directly bonded to silicon one or more hydrogen atoms, halogen atoms, or groups of formula -OR1, wherein Rl represents a hydrogen atom or a hydrocarbon moiety. the silicon-organic compound acting as a hydrophobic agent which is capable of rendering the metal phosphide hydrophobic.

wherein each of the R2 groups independently represent an alkyl group or an aryl group, the groups x, y and z having the same meaning as R2 or being hydrogen, but with the proviso that not all of the said groups x, y and z are groups of formula R2, whilst n, m and o each independently represent a whole number, the total sum of n, m and o being from 10 to 1000.

9. A composition according to Claim 8, wherein R2 represents an alkyl group having a molecular weight not exceeding 150, or a phenyl group.

10. A composition according to Claim 9, wherein the alkyl group is methyl or ethyl.

11. A composition according to any one of Claims 8 to 10, wherein the total sum of n, m and o is from 20 to 100.

12. A composition according to any one of Claims 8 to 11, wherein the substituents R2 in the silicon-organic compound are all the same.

13. A composition according to any one of the preceding claims, wherein the silicon-organic compound is a methyl-hydrogen polysiloxane.

14. A composition according to any one of the preceding claims, where the silicon-organic compound is one having a viscosity at room temperature of from 10 to 100 cP.

15. A composition according to any one of the preceding claims, containing from 0.1 to 3% by weight of the silicon-organic compound.

16. A composition according to Claim 15, containing from 0.2 to 0.5% by weight of the silicon-organic compound.

17. A composition according to any one of the preceding claims, wherein the metal phosphide is calcium phosphide, magnesium phosphide or aluminium phosphide.

18. A composition according to any one of Claims 2 to 17, wherein all or a substantial part of the silicon-organic compound is chemically bonded to surface of the particulate phosphide.

19. A composition according to Claim 18, wherein the particulate phosphide is a technical grade aluminium phosphide of the type conventionally used in pesticidal aluminium phosphide preparations.

20. A composition according to Claim 18 or Claim 19, wherein the presence of the silicon-organic compound is substantially restricted to the surfaces of the particulate phosphide.

21. A composition according to Claim 1, substantially as described in any one of the foregoing Examples and Experiments.

22. Process for the manufacture of a metal phosphide composition as claimed in any one of Claims 2 to 21, which process comprises mixing the particulate phosphide with the silicon-organic compound and treating the resultant mixture so that the organo-silicon compound becomes chemically bound to the surface of the metal phosphide particles thereby rendering the said surface hydrophobic.

23. Process according to Claim 22, wherein the organo-silicon compound is caused to react by applying heat to the mixture and/or by the presence of a suitable catalyst.

24. Process according to Claim 23, wherein the mixture is heated to from 800 to 200"C.

25. Process according to Claim 24. wherein the mixture is heated to from 110 to 150"C.

26. Process according to Claim 23, 24 or 25, wherein the catalyst is ammonia, an ammonium compound or amine compound.

27. Process according to Claim 26, wherein the catalyst is present in an amount of 0.5 to 3% by weight based on the weight of the metal phosphide.

28. Process according to Claim 26 or 27, wherein the catalyst is ammonium carbonate or ammonium carbamate.

29. Process according to Claim 23, 24 or 25, wherein the catalyst is an organo-metallic compound.

30. Process according to Claim 29, wherein the organo-metallic compound is a compound of lead, tin, zirconium, cobalt or titanium.

31. Process according to Claim 30, wherein the organo-metallic compound is a zirconium butoxide, dibutyl tin dilaurate and/or di-n-octyl tin maleinate.

32. Process according to Claim 29, 30 or 31, wherein the amount of organo-metallic compound is 1 to 10% by weight based on the weight of the silicon-organic compound.

33. Process according to any one of Claims 22 to 32, wherein the particulate phosphide is mixed only with the silicon-organic compound, where appropriate with a subordinate catalysing amount of catalyst, until the silicon-organic compound has become wholly or substantially wholly bonded to the phosphide particles which are thereby rendered hydrophobic.

34. Process according to Claim 22, substantially as described in any one of the foregoing Examples and Experiments.

35. A metal phosphide composition according to any one of Claims 2 to 21 whenever prepared by a process as claimed in one of Claims 22 to 34.

36. A pest control composition comprising a metal phosphide composition as claimed in any one of the claims 1 to 21 and 35, and an ammonium compound and/or an amine compound for suppressing the self-ignition of hydrogen phosphide.

37. A composition according to Claim 36, wherein the ammonium and/or amine compounds are selected from urea and ammonium carbamate.

38. A composition according to Claim 36 or 37, wherein the ammonium and/or amine compound is a mixture of particles of urea and particles of ammonium carbamate.

39. A composition according to any one of Claims 36 to 38, in the form of bodies moulded by compression from the mixture of particles.

40. A composition according to Claim 36 substantially as hereinbefore described.

41. A composition according to Claim 36, substantially as hereinbefore described in any one of the foregoing Examples and Experiments.

42. Technical metal phosphide, reactive with water to generate hydrogen phosphide and comprising surface bonded entities of the general formula MeO- SI wherein Me represents a metal atom of the metal phosphide and the group - Si represents a hydrophobic silicon-organic group.

43. Metal phosphide according to Claim 42, wherein the group - Si is represented by a group of the general formula

wherein each of the R2 groups independently represents an alkyl or aryl group, the groups x, y and z having the same meaning as R2 or being hydrogen subject to the conditions that not all of the groups x, y and z are groups of formula R2, and wherein n, m and o each independently represents a whole number, the total sum of n, m and o being from 10 to 1000 and wherein at least one of the groups x, y and z represents the group MeO-.

44. Metal phosphide according to Claim 43, wherein R2 represents methyl, ethyl or phenyl.

45. Metal phosphide according to Claim 43 or 44, wherein the sum total of n, m and o is from 20 to 100.

46. Metal phosphide according to Claim 42, substantially as hereinbefore described.

47. A method of pest control which comprises exposing material or an environment in which pests are to be controlled to a pesticidal concentration of hydrogen phosphide generated by the action of moisture on a metal phosphide or composition thereof as claimed in any one of Claims 1 to 21, or 36 to 41 or 42 to 46.

48. Method according to Claim 47, wherein the moisture is atmospheric moisture.

49. Method as claimed in Claim 47 or 48, wherein the said phosphide or composition thereof is added to agricultural produce which is to be disinfested.

50. Agricultural produce whenever disinfested by a method as claimed in Claim 49.